O-18 O-16 MAPPING AND HYDROGEOLOGY OF A SHORT-LIVED (APPROXIMATE-TO-10 YEARS) FUMAROLIC (GREATER-THAN-500-DEGREES-C) METEORIC-HYDROTHERMAL EVENT IN THE UPPER PART OF THE 0.76 MA BISHOP TUFF OUTFLOW SHEET, CALIFORNIA/

O-18/O-16 data from the 200-m-thick, 0.76 Ma Bishop Tuff outflow sheet provide evidence for a vigorous, short-lived (approximate to 10 years ), high-temperature, fumarolic meteoric-hydrothermal event. This is pr oved by: (1) the juxtaposition in the upper, partially welded Bishop T uff of low-O-18 groundmass/glass (delta(18)O = -5 to +3) with coexisti ng quartz and feldspar phenocrysts having magmatic delta(18)O values ( +8.7 +/- 0.3; +7.5 +/- 0.3); and (2)the fact that these kinds of O-18/ O-16 signatures correlate very well with morphological features and ma pped zones of fumarolic activity. Profiles of delta(18)O With depth in the Bishop Tuff within the fumarole area define a 40- to 50-m-thick, low-O-18, stratigraphic zone that is sandwiched between the essentiall y unwelded near-surface portion of the tuff and an underlying, densely welded black tuff that displays magmatic O-18/O-16 values. Shallow-di pping columnar joints and other fumarolic features (i.e., subhorizonta l tubular conduits and steep fissures) correlate very well with these pervasively devitrified, low-O-18 zones. The base of the low-O-18 zone is extremely sharp (similar to 3 parts per thousand per meter) and is located directly above the transition from partially welded tuff to d ensely welded black tuff. The observed average whole-rock O-18-depleti ons within this low-O-18 zone are about 6-7 parts per thousand, requir ing meteoric water/rock ratios in excess of 0.24 in mass units. Rainfa ll on the surface of the tuff would not have been high enough to suppl y this much H2O in the short lifetime of fumarolic activity, suggestin g that some recharge must have been from groundwater flow through the upper part of the tuff, above the sloping (1 degrees-5 degrees) top of the impermeable lower zone. This is compatible with the observation t hat the fumarolic areas roughly correlate with the preeruptive regiona l drainage pattern. Some of this recharge may in part have been from t he lake that filled Long Valley caldera, which was dammed by the Bisho p Tuff up to the level of this boundary between the partially and dens ely welded zones (approximate to 7000 ft, the elevation of the highest Long Valley Lake shorelines). Gazis et al. had previously shown that the 2.8-Ma intracaldera Chegem Tuff from the Caucasus Mountains exhibi ts exactly the same kind of O-18-signature that we have correlated wit h fossil fumaroles in the Bishop Tuff outflow sheet. Although not reco gnized as such by McConnell et al.; O-18/O-16 data from drill-hole sam ples from the intracaldera Bishop Tuff in Long Valley also display thi s characteristic O-18 signature (i.e., analogous delta(18)O-depth prof iles, as well as low-O-18 groundmass coexisting with high-O-18 feldspa r phenocrysts). This fumarolic O-18/O-16 signature is observed to much greater depths (approximate to 650-750 m) in the intracaldera tuffs ( approximate to 1500 m thick) than it is in the approximate to 200-m-th ick Bishop Tuff outflow sheet (approximate to 80 m depth). (C) 1998 El sevier Science B.V. All rights reserved.